Process for extrusion of tubular plastic film with external and internal spray cooling



March 28, 1967 E. RINGLEY ETAL 3,311,682

PROCESS FOR EXTRUSION OF TUBULAR PLASTIC FILM WITH EXTERNAL AND INTERNALSPRAY COOLING Filed Oct. 21, 1965 2 Sheets-Sheet 1 (III/J INVENTORSEdward Ringly Edward J. Grazulis ATTORNEY March 28, 1967 E. RENGLEY ETAL5 PROCESS FOR EXTRUSION OF TUBULAR PLASTIC FILM WITH EXTERNAL ANDINTERNAL SPRAY COOLING Filed Oct. 21 1963 INVENTQRS,

Edward Ring]? Edward J. Grazulis ATTORN Y United States Patent PROCESSFOR EXTRUSION OF TUBULAR PLASTIC FILM WITH EXTERNAL AND INTERNAL SPRAYCOOLING Edward Ringley, Wycoif, and Edward J. Grazulis, Clifton,

N.J., assignors, by mesne assignments, to Gulf 0i Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 21, 1963, Ser.No. 317,711 1 Claim. (Cl. 264-95) This invention relates to improvedthermoplastic polymer extrusion processes.

Thermoplastic polymer films, particularly films prepared fromcrystalline thermoplastic polymers such as polyethylene andpolypropylene, constitute one of the largest outlets for thermoplasticpolymers. Notwithstanding the fact that a prodigious volume of work hasbeen carried out on film-manufacturing processes, it is recognized thatprocesses for the manufacture of thermoplastic polymer films stillsufi'er from a number of shortcomings. One of the primary deficienciesof existing processes is that they are relatively expensive.Specifically, most films cost at least twice as much as thethermoplastic polymers employed in their manufacture. The lowest-costprocess currently employed in manufacturing thermoplastic polymer filmsis the well-known blow-extrusion or trapped-bubble process. The majorfactors which hurt the economics of the blow-extrusion process are thehigh capital cost per unit of productive capacity and the substantialpercentage of the manufactured film which fails to meet criticalmanufacturing specifications. Another deficiency of existing filmprocesses, particularly the blow-extrusion process, is that the filmso-produced is deficient in certain respects.

We have now discovered a novel, highly flexible extrusion process whichis decidedly superior to the currentlyused blow-extrusion filmprocesses. Specifically, the cost of converting thermoplastic polymer tofilm is substantially lower and, in addition, the film so-produced isdistinctly superior to film prepared by currently-employedblow-extrusion processes.

The present invention, in its broadest aspect, comprises a criticalcombination of steps in which a thermoplastic polymer is extruded as atube at an elevated temperature, e.g. above about 270 F., at a rate ofat least 8 lbs/hr. per circumferential inch of die-opening and theextruded tube, within about 4 die diameters downstream from the die, iscontacted with a thin spray of cooling liquid such as water whichcontacts its entire exterior circumference in a lane substantiallytransverse to the direction of extrusion. The cooling liquid cools thehot tube to below about 180 F. within 1 second. The rapid rate at whichthe tube is cooled permits the process to be operated at very highspeeds and substantially lowers the cost of producing polymer film. Thefilm produced by the process of this invention, particularly filmprepared from low-density polyethylene, has greatly improved physicaland optical properties which will be subsequently described in greaterdetail.

The nature of the invention and the advantages thereof will be betterunderstood from the following detailed description thereof when read inconjunction with the drawings in which:

FIG. 1 is a vertical view, partially in section, of one embodiment ofthe invention;

FIGV 2 is a view taken through line 22 of FIG. 1;

FIG. 3 is a vertical view, partially in section, of another embodimentof the invention; and

FIG. 4 is a view taken through line 44 of FIG. 3.

Referring to FIGS. 1 and 2, hot polymer, such as lowdensitypolyethylene, is fed from a conventional screw extruder into aconventional blow-extrusion die 12.

ice

The hot polymer is extruded in the form of a tube 14 and, after beingcooled as subsequently described, is collapsed by rolls 15 and passesthrough nip rolls 16 which seal tube 14. Nip rolls 16 also function todraw tube 14 from die 12 to reduce the wall thickness of the tube andorient the polymer in the machine direction. Air at a pressure on theorder of 0.1-2.0 p.s.i.g. is introduced into tube 14 through line 18 andexpands the tube. The expansion of the tube reduces the wall thicknessof tube 14 and orients the film transversely to the machine direction.The operations above-described are conventional in blow-extrusionprocesses.

Approximately one die diameter downstream from the face of die 12, tube14 has its exterior surface shockcooled by cooling liquid 22, deliveredfrom a cored liquid cooling ring 24. Cooling ring 24 can be raised andlowered in a vertical plane by support means not shown. The coolingliquid rapidly cools the temperature of tube 14 to below about F. withinless than 1.0 second. When the extruded polymer is low-densitypolyethylene or another crystalline polymer, the temperature of theextruded tube can be measured by observing the appearance of frostline26. With low-density polyethylene, the frostline appears when thetemperature is reduced to about 180 F.

The cored liquid cooling ring is generally circular in cross-section andresembles a doughnut in appearance. In its interior face is provided a2-20 mil, and preferably an 8 mil, slit opening 28. Water under pressureis introduced into cooling ring 24 from line 30 and is forced out of thecooling ring through slit opening 28 at a high velocity. The coolinliquid as it is forced from opening 28 forms a continuous liquid sheetwhich is best seen in FIG. 2. The sheet of cooling liquid, on contactingtube 14, is broken up and forms a turbulent column of liquid 32 whichinitially contacts tube 14. Tube 14 is cooled very rapidly and frostline26 appears within about 1 inch of the point at which the water sheetcontacts tube 14.

The cooling liquid which fiows down tube 14 is collected in trough 36and is recirculated. The water is withdrawn from trough 36 by means ofline 38 and pump 40. A filter 42 is included upstream in line 30 toremove any suspended particles whose maximum diameter exceeds 10microns. A valve 4 is provided in line 30 to further control the volumeof cooling liquid delivered to cooling ring 24. As needed, makeup wateris intro duced into trough 36 by means of a water line not shown.

The embodiment of the invention illustrated in FIG. 3 is generallysimilar to that illustrated in FIG. 1 except that second cooling meansare positioned within tube 14 to shock-cool the interior surface of tube14. This secondary cooling means consists of a cored circular disc 7 50having a slit opening 52 provided in its face adjacent to tube 14.Cooling liquid under pressure is fed through line 54 into disc 50 and isforced through slit opening 52 as a continuous liquid sheet whichcontacts the interior surface of tube 14. Cored circular disc 50 andexterior cored cooling ring 24 are positioned so that the coolingliquids forced therefrom Contact tube 14 in the same transverse plane.

The liquid forced from disc 52 collects into a pool 54 which iswithdrawn through line 56 by pumping means not shown. Tube 14 isexpanded by air which is introduced through line 18. A series of tubularpassageways 58 are provided in disc 50 so that the air may freely passtherethrough to maintain tube 14 in an inflated condition downstream ofdisc 52, If desired, the embodiment of FIG. 3 can be modified byremoving exterior cooling ring 24 in which event tube 14 is cooledsolely by the cooling liquid expressed from disc 50.

The following examples are set forth to illustrate more clearly theprinciple and practice of this invention to those skilled in the art.Where parts or quantities are mentioned, they are parts or quantities byweight unless othervention, as compared with the corresponding filmsprepared from the same resin by conventional blow-extrusion methodswill, in virtually all instances, have better transparency, lower haze,better gloss, higher impact strength Wise notedand a lower density.

Examples LXI The above descriptions and particularly the drawings ASeries f 11 il d 5 il fil f l density l and the examples are set forthby Way of illustration only. ethylene are prepared employing theapparatus of FIG. 1. Many other variations and modifications thereofwill be The extrude-r used has a 2 inch internal diameter and anapparent to those skilled in the art and can be employed L/D ratio of :1and a polyethylene metering screw. 10 without departing from the spiritand scope of the inven- The die employed has a 3 inch diameter and a lipopening tion herein described. of 0.025 inch. The cored cooling ring hasan 8 inch in- What is claimed is: ternal diameter and an 8 mil slitopening. The distance In a blow-extrusion process for preparing thingauge between the die face and the slit opening of the cooling film ofan ethylene homopolymer in which the ethylene ring is 6 inches.homopolymer is extruded at a rate of at least about eight The resinemployed, the process conditions employed, pounds per hour percircumferential inch of die opening the diameter of the expanded bubbleand the gauge of the and at a temperature above about 270 F. intoambient film are set forth in Table I. air in tubular form, the extrudedtube is withdrawn from TABLE 1 Example N0. I II III IV V VI VII VIII IXX XI Resin Resin Resin Resin Resin Resin Resin Resin Resin Resin ResinResin A1 A! B2 B2 (3: C5 1 D4 E5 H6 A1 Process Conditions:

Tem Feed Zone 400 400 400 400 400 400 300 300 350 400 275 Temp, DeliveryZone 450 450 450 150 450 450 325 325 400 475 375 'leinp., Die Zone 425425 425 425 425 425 325 325 425 450 350 Melt Temperature..- 455 455 450465 405 330 330 400 480 375 Screw Speed, r.p.ni 10s 10s 10s 10s 10s 10s10s 10s 10s 108 Wind Up Speed, minim 05 70 97 75 75 75 70 93 300Extrusion Rate. lbs/hr 92 92 96 96 94 94 96 96 75 S9 S8 Cooling WaterTemp, 1 125 125 125 125 125 125 120 Diameter Expanded Bubble, inches 3.9 5.1 3. 9 5.1 3. 0 5.1 5.1 5.1 3. 9 3. 9 5.1 Film Gauge, inch 0. 005 0.005 0.005 0. 005 0.005 0. 005 0. 005 0. 005 0. 005 0. 005 0.00125 1Polyethylene having a density of 0.918 and a melt index of 0.5.Polyethylene having a density of 0.926 and a melt index of 2.0.

'- Polyethylene having a density of 0.923 and a melt index of 0.8. 5Polyethylene having a density of 0.924 and a melt index of 1.2.

3 Polyethylene having a density of 0.922 and a melt index of 0.5Polyethylene having a density of 0.918 and a melt index of 0.5.

A series of 11 control resins are prepared under the the die underpositively applied force to orient the ethylene identical conditions setforth in Table I, except that the homopolymer in the directionperpendicular to the die resin is not contacted with water, but ratheris cooled with face, gas pressure is applied to the interior of theextruded a conventional air cooling ring, A necesary reduction is tubeto expand the tube and orient the ethylene homopolymade in the rate ofextrusion so that the expanded tube 45 mer in the direction parallel tothe die face, the extruded would not fuse together when passed throughthe nip tube is cooled, collapsed and passed through nip rolls, rolls.said cooling accomplished within a distance of about four In eachinstance, the films prepared by the process of die diameters downstreamof the die face, by directing a the invention, as compared with thecontrol process, have stream of water substantially perpendicularlyagainst the strikingly greater transparency, strikingly less haze, mate-50 exterior surface of the hot expanded tube and cooling the riallybetter gloss, materially better impact strength and a hot tube to belowabout 180 F. within less than about lower density. one second, saidstream of water, at least immediately While the examples and thedrawings illustrate the prior to contacting the expanded tube,constituting a solid manufacture of blown-film, the process of theinvention is sheet in a plane substantially parallel to the die face,the not limited to processes in which the extruded tube is ex- 55improvement which comprises also directing a stream of named In someinstances, p i ly in the n a water substantially perpendicularly againstthe interior surture of heavy wall tubing for use in the manufacture offace of the hot extruded tube, said interior stream of industrial bagsand the like, it is not necessary to expand water contacting the hotextruded tube in substantially the the tube. same plane as the exteriorstream of water so that heat is y thermoplastic resin susceptible toConversion into 60 withdrawn substantially uniformly from both surfacesof film can be employed in the process of the present inventh hot extr dd t be, tion. By following the procedures described herein, highqualityfilm has been produced from nylon 6, polypropyl- References Cited by theExaminer ene and linear polyethylene in addition to the high-pres-UNITED STATES PATENTS sure polyethylene as illustrated in the examples.65 2 955 321 10 1960 Former et 1 1 14 XR As noted in the examples,polyethylene film obtained 2 9 7 7 7 /19 1 Berry et 1 13 15 by themethod of this invention, as compared with film 3 090 993 5 19 3Heisterkamp et 1 1g 14 XR prepared from the same polyethylene resin by aconven- 3 011 2 1965 Cheney et 1 13 14 tional blow-extrusion process,has a lower density, better 3 193 547 7 19 5 Schott 264 .2Q9 strength atthe crease line, higher impact strength, higher 70 3 207 323 9 19 5(Hyde at 18.44 tensile strength in the cross-machine direction andbetter 3 226 459 12 19 5 Tijunelis 264 21 XR optical clarity. Thereasons why such improved properties are obtained are not obvious.

The polymer films prepared by the method of this in- ROBERT F. WHITE,Primary Examiner.

L. S. SQUIRES, A. R. NOE, Assistant Examiners.

